Holograms are often employed to provide some degree of document security. Many bankcards carry a holographic image including an image of the authentic card user so that the identity of that user can be verified. Holograms are also imbedded within security documents so that they are invisible to the unaided eye. To verify or authenticate such documents, the hologram is irradiated with light of a suitable wavelength. Depending on the wavelength used, the holographic image can either be viewed directly or it can be sensed using suitable imaging techniques. While holograms provide an initial level of security, the techniques to produce holograms are becoming readily available such that a hologram may be copied thereby limiting the value of holograms. Conventional watermarks such as the images of a manufacturer's logo that are pressed onto paper or the watermarks of currency notes can also be reproduced.
For documents distributed electronically, digital watermarks have been employed. A digital watermark may be an invisible signal that is overlaid into an electronic file. The overlay may contain critical information or hidden information which is only retrievable by the rightful recipient in position of the proper decoder. A digital watermark may be imbedded in an electronic document. When someone attempts to copy and use the electronic document, the digital watermark is copied therewith and is evidence that the document was copied from the original. Alternatively, alteration of a document can destroy the digital watermark and make the content invalid.
Conventional optical watermarks use optical devices such as photocopiers to retrieve the watermark. An optical watermark can be a combination of an organization's logo and words to indicate ownership of a document. If there is an attempt to photocopy a printed document with the optical watermark, the copied document will show the watermark illustrating that the document is not the original. Optical watermarks are particularly useful to protect print documents from unauthorized reproduction.
While optical watermarks that rely upon optical devices such as photocopiers to retrieve the watermark are suitable for loose paper documents, a need remains for security devices applied to paper or plastic substrates such as those used in packaging for retail products. A consumer seeking assurances that a packaged product was actually produced by a particular manufacturer may not have access to optical devices for testing the packaging of a product.